1. Field of the Invention
The present invention relates to an inverter apparatus for converting DC power generated by an independent DC power supply such as a solar cell into AC power and supplying the power to home- and business-use general AC loads or to existing commercial power systems and the like.
2. Description of the Prior Art
A conventional inverter apparatus is composed of an inverter bridge made up of several switching devices, a transformer for electrically insulating the DC power source from commercial power systems or loads, a low-pass filter, and a control circuit for performing ON/OFF control on the plurality of switching devices constituting the inverter bridge. As the above transformer, there have been used power-frequency transformers or high-frequency transformers intended for the miniaturization of the apparatus.
First, a conventional example of the inverter apparatus using a power-frequency transformer is described with reference to FIG. 14. DC power outputted from a solar cell 2 is inputted to an inverter apparatus 1. The input DC power is converted into AC power by an inverter bridge 32, and supplied to a commercial power system 3 via a power-frequency transformer 33 which is provided at an output end of the inverter apparatus 1 in order to insulate the solar cell 2 from the commercial power system 3. A DC capacitor 12 for suppressing the input power fluctuating of the inverter apparatus 1 and a DC input current detector 13 are connected in the preceding stage of the inverter bridge 32. An AC filter 16 for removing harmonic components of AC current and an inverter output current detector 14 are connected in the succeeding stage of the inverter bridge 32. Further, an interconnection relay 15 is provided in the succeeding stage of the AC filter 16, whereby interconnection and disconnection with the commercial power system 3 is implemented.
A control circuit 34 of the inverter apparatus 1 is composed of a gate drive circuit 35, a PWM (Pulse Width Modulation) control unit 36, an error amplifier 37, a carrier signal generator 38, a signal processing unit 39, a sine-wave signal storage unit 40, an A/D converter 41, and a D/A converter 42.
The PWM control unit 36 generates a first pulse train signal obtained by comparing a sine-wave signal equal in frequency (50/60-several hundreds Hz) to the voltage waveform of the aforementioned commercial power system 3 with a high-frequency (about 10/kHz, but not lower than 10 kHz) carrier signal synchronized with the sine-wave signal, a second pulse train signal obtained by inverting the first pulse train signal, a third pulse train signal obtained by comparing an inverted carrier signal, which is the inversion of the aforementioned carrier signal, with the sine-wave signal, and a fourth pulse train signal obtained by inverting the third pulse train signal. FIGS. 15B, 15C, 15D and 15E show respective waveforms of these pulse train signals. It is noted that in FIGS. 15A, 15B, . . . , 15F, high-frequency waves of about 10 kHz, but not lower than 10 kHz are schematically illustrated.
These first to fourth pulse train signals are inputted to the gate drive circuit 35. Based on these signals, gate drive signals equal in frequency to the carrier signal are generated for four switching devices Q1 to Q4 constituting the inverter bridge 32. With the gate drive signals, the switching devices Q1 to Q4 are controlled to be turned on and off at the same frequency as that of the carrier signal. As a result, an output pulse train waveform Ei as shown in FIG. 15F is produced by the inverter bridge 32. Further, the output waveform Ei is subjected to a harmonic-component removing process and a smoothing process by the succeeding-stage AC filter 16, resulting in a 50/60-several hundreds Hz sine-wave AC output. The sine-wave AC output undergoes the input-output insulation by the power-frequency transformer 33 and thereafter is inputted to the commercial power system 3. In this case, the power-frequency transformer 33 is excited at a frequency of the 50/60-several hundreds Hz sine-wave AC output.
The A/D converter 41 converts a DC voltage signal V.sub.in and a DC current signal I.sub.in, which are analog signals derived from the solar cell 2, and a utility line voltage signal V.sub.out into digital quantities, and then transmits the resulting signals to the signal processing unit 39. In order to maximize the output power from the solar cell 2, the signal processing unit 39 performs a maximum power point tracking operation, by which the solar cell operating point is made coincident with a maximum point on the solar cell output characteristic curve. The signal processing unit 39 also reads out a sine-wave signal (50/60-several hundreds Hz) which serves as a current command value for controlling the inverter apparatus 1, from the sine-wave signal memory unit 40 where a plurality of sine-wave signals having different amplitudes are previously stored. The sine-wave signal storage unit 40 is normally storing the aforementioned sine-wave signals having a plurality of different amplitudes proportional to the amplitude of rated output current waveform of the inverter apparatus 1, as digital quantities quantized in the unit of half the period or one period and every certain time intervals. The D/A converter 42 converts a read sine-wave signal into an analog signal and then transmits it to the error amplifier 37. The error amplifier 37 receives as inputs an inverter output current signal Iout derived from the inverter output current detector 14, and the aforementioned sine-wave signal. The error amplifier 37 compares the two signals with each other to determine an error, and outputs a reference wave signal obtained by amplifying the error, to the PWM control unit 36. The carrier signal generator 38 outputs similarly to the PWM control unit 36 a carrier signal (higher than ten kHz) synchronized with the sine-wave signal. As a result, the output current of the inverter apparatus 1 undergoes a change in response to the sine-wave signal that serves as a current command value. In this case, when the output current of the inverter apparatus 1 is controlled with the above sine-wave signal, AC power with a power factor of 1 can be supplied to the existing commercial power system 3, by providing a sine-wave signal of the same phase and the same frequency (50/60-several hundreds Hz) as the voltage of the commercial power system 3.
Next described is a case where a high-frequency transformer is used. An apparatus employing a power-frequency transformer is disadvantageous in reducing the size and weight of the inverter apparatus because of the large weight and capacity of the power-frequency transformer. With the use of a high-frequency transformer, on the other hand, such problems can be solved. When a high-frequency transformer is used, the transformer needs to be excited at high-frequency voltage. An example using the current instantaneous value control method developed for this purpose is described below with reference to FIG. 16.
An inverter apparatus 100 is inserted between a solar cell 2 and an existing commercial power system 3. The inverter apparatus 100 converts DC power generated by the solar cell 2 into AC power of 50/60 Hz, and supplies the power to loads in grid-connection with the commercial power system 3. In the inverter apparatus 100, the input DC power is converted into high-frequency alternating voltage under the current instantaneous value control by a high-frequency inverter bridge 4 made up of switching devices Q1 to Q4, and fed to the primary side of a high-frequency transformer 5. The high-frequency alternating current is rectified by a diode bridge 6 on the secondary side of the high-frequency transformer 5, and subjected to a harmonic-component removing process and a smoothing process by a filter circuit made up of a DC reactor 7 and a capacitor 47 connected in parallel therewith. Further, the processed current is converted into AC power of commercial frequency under polarity reversing control by a low-frequency inverter bridge 8 made up of switching devices S1 to S4. Then, the power is supplied to the commercial power system 3 via an interconnection relay 55 and an AC filter 16.
A signal processing unit 43 receives as inputs a voltage signal V.sub.in of the solar cell 2, a current signal I.sub.in detected by a DC input current detector 13, a current (inverter output current) signal I.sub.t on the primary side of the high-frequency transformer 5 detected by an inverter output current detector 14, and a voltage signal V.sub.out of the commercial power system 3. The signal processing unit 43 produces as outputs a current command signal and a polarity decision signal. A hysteresis comparator 44 receives as inputs a primary-side current I.sub.t of the high-frequency transformer 5 detected by the inverter output current detector 14 and the aforementioned current command signal. The hysteresis comparator 44 performs via a NOT circuit 45 the control of alternately turning on and off the switching devices Q1, Q4 and Q2, Q3 that constitute the high-frequency inverter bridge 4, so that the primary-side current of the high-frequency transformer 5 is repeatedly reciprocated within a range of a constant width having upper and lower limit values around the current command signal. More specifically, with respect to the current command signal (I.sub.REF) as shown in FIG. 17, an upper limit value I.sup.+ and a lower limit value I.sup.- with a specified width .DELTA.I are previously given to the hysteresis comparator 44 as set values. Then, the primary-side current signal I.sub.t of the high-frequency transformer 5 in FIG. 16, which is the actual value of the control quantity, is detected by the inverter output current detector 14, and fed to the hysteresis comparator 44 together with the current command signal. When the current signal I.sub.t, which is the actual value of control quantity, exceeds the upper limit set value I.sup.+ of FIG. 17 (I.sup.+ =I.sub.REF +.DELTA.I), the switching devices Q1, Q4 of the high-frequency inverter bridge 4 of FIG. 16 are turned off while the switching devices Q2, Q3 are turned on via the NOT circuit 45, so that the current gradient is turned into a decrease. On the other hand, when the current signal I.sub.t of FIG. 17 decreases below the lower limit set value I.sup.- (I.sup.- =I.sub.REF -.DELTA.I), the switching devices Q1, Q4 are turned on while the switching devices Q2, Q3 are turned off, so that the current signal I.sub.t increases. By performing such switching control, the actual value of the current signal I.sub.t transits reciprocatingly between I.sup.+ and I.sup.- each time the switching operation is effected. In this operation, if a sine-wave signal having the same frequency as the commercial power system 3 and having an arbitrary amplitude is used as the current command signal (I.sub.REF), the current signal I.sub.t changes repeatedly and reciprocatingly responsive to even very fast switching operation within a range of .+-..DELTA.I around the current command signal. Thus, a sine-wave current waveform having a commercial frequency and having an amplitude proportional to that of the current command signal can be obtained. As described above, the primary-side current of the high-frequency transformer 5 in the inverter apparatus 100, i.e., the magnitude of the inverter output current can be controlled by the amplitude of the current command signal (I.sub.REF).
The fold-back control circuit 46 receives as an input the aforementioned polarity decision signal and alternately switches the turn-on and -off of the switching devices S1, S4 and S2, S3 that constitute the low-frequency inverter bridge 8, according to the polarity of the voltage signal V.sub.out of the commercial power system 3. By this control, DC power rectified into a full-wave rectified current by the diode bridge 6 is formed into a sine-wave AC output at the succeeding stage of the low-frequency inverter bridge 8.
For reduction in size and weight of the inverter apparatus, a high-frequency transformer is preferably used. The reason is that the high-frequency transformer results in about 1/30 the capacity and about 1/20 the weight of the power-frequency transformer.
However, the above current instantaneous value control method used as a method for exciting the high-frequency transformer with high-frequency voltage, superior as it is, has difficulties in optimizing the setting of the upper and lower limit values of the hysteresis width for the control method. Too large set values result in an increased distortion while too small set values result in a decreased width of the pulse train signal obtained through a comparison between the current command value and the inverter output current. This causes the control system to be more unstable than in the PWM control using the low-frequency transformer. A further problem is that seeking a more stable control system would lead to a more complex control circuit.